The present invention relates to an indication display device of a finder optical system used in a camera for photography or an electronic camera.
A finder for a camera needs indication display within the field of view to display an indication of a distance measurement range for autofocusing and an indication for parallax correction in photography at the closest focusing distance. Conventionally, a real-image finder is provided with an indication display member near an intermediate image formation plane. Known indication display methods include a method wherein incident light is blocked to display an indication, and a method wherein a wedge-shaped deflecting member is provided on the surface of an optical element of a Porro prism to deflect incident light in a direction in which it does not reach a viewer's pupil, as disclosed in Gazette Containing the Japanese Patent No. 2629690.
However, the method wherein incident light is blocked requires extra processing, e.g. vapor deposition onto a transparent member. This causes an increase in the cost. In the case of the method wherein incident light is deflected, if an indication display member is formed as a molded part, it is unnecessary to subject each individual parts to extra processing, and there is no increase in the cost. However, in the case of a prism in which an entrance surface and an exit surface are disposed close to each other as in a pentagonal prism, for example, deflected rays may be totally reflected by the exit surface to reach the viewer's pupil as light still having a strong intensity, thus causing ghost light. Rays deflected by a deflecting member for indication deviate from a visual field viewing light path but are still directed toward the pupil. In a Porro prism such as that shown in FIG. 1, an entrance surface 11 is surrounded with a first reflecting surface 12 and three diffusing surfaces 13. Therefore, most of deflected rays impinge on the diffusing surfaces 13 to become diffused light of weak intensity. Accordingly, only a slight amount of light reaches the viewer's pupil and is not recognized as ghost light. In the case of a pentagonal prism such as that shown in FIG. 2, light that travels along a normal light path enters the prism through an entrance surface 21 and is successively reflected by reflecting surfaces 23 and 23' to exit from an exit surface 22. However, the exit surface 22 has a mirror surface not included in the normal light path in the neighborhood of the entrance surface 21. Therefore, as shown in part (a) of FIG. 4, deflected rays 40 may be totally reflected by an exit surface 41 (22) and reflected by a first reflecting surface 42 and a second reflecting surface 43 to reach a viewer's pupil without being diffused. In this case, a bright ghost image having a size equivalent to the indication is undesirably observed, giving rise to a problem.